Immunoglobulin binding domains find therapeutic use in a wide variety of formats, including the traditional antibody format of a homodimeric immunoglobulin heavy chain associated with a cognate light chain. Many of these formats, including the traditional format, exhibit pharmacokinetic features in vivo that are suboptimal, due to a wide variety of factors. In recent decades, disparate approaches have been tried to improve pharmacokinetics. These include, e.g., increasing hydrodynamic radius to reduce renal clearance by conjugation to polymers (e.g., PEG; reviewed in, e.g., Duncan, R. (2006) Polymer conjugates as anticancer nanomedicines, Nat. Rev. Cancer 6:688-701); sialylation of N-glycans (reviewed in, e.g., Stork, R. et al. N-glycosylation as novel strategy to improve pharmacokinetic properties of bispecific single-chain diabodies, J. Biol. Chem. 283(12):7804-7812); Fc modifications for promoting neutral pH Fc-FcRn binding while promoting release at endosomal pH and association with serum albumin (see, e.g., Chuang et al. (2002) Pharmaceutical Strategies Utilizing Recombinant Serum Albumin, Pharm. Res. 19(5):569-577). In appropriate applications and for appropriate formats, each of these approaches may offer some benefits.
However, there remains a need in the art for improving therapeutic effects and modalities for biopharmaceuticals, including but not limited to manipulating immunoglobulin variable domain structures to engineer variable domains that exhibit pH-dependent binding. There is a need for variable domains for use in antigen-binding proteins of a variety of formats, wherein the variable domains (or antigen-binding fragments thereof) confer upon the antigen-binding protein pH sensitivity with respect to binding a target antigen or receptor. There is also a need in the art for systems and methods for generating pH-dependent immunoglobulin variable domains and antigen-binding fragments thereof. There is a need for biological systems that can generate a wide diversity of immunoglobulin variable domains, wherein the wide diversity is enriched with respect to titratable amino acids that may confer upon the variable domain pH sensitivity, e.g., the ability to bind a target antigen or epitope at one pH (e.g., a neutral, or high pH), yet release the target antigen or epitope at a second pH (e.g., a low, or endosomal, pH).
Immunoglobulin light chains in certain formats present unique challenges. Antibodies typically comprise a homodimeric heavy chain component, wherein each heavy chain monomer is associated with an identical light chain. Antibodies having a heterodimeric heavy chain component (e.g., bispecific antibodies) are desirable as therapeutic antibodies. But making bispecific antibodies having a suitable light chain component that can satisfactorily associate with each of the heavy chains of a bispecific antibody has proved problematic.
In one approach, a light chain might be selected by surveying usage statistics for all light chain variable domains, identifying the most frequently employed light chain in human antibodies, and pairing that light chain in vitro with the two heavy chains of differing specificity.
In another approach, a light chain might be selected by observing light chain sequences in a phage display library (e.g., a phage display library comprising human light chain variable region sequences, e.g., a human scFv library) and selecting the most commonly used light chain variable region from the library. The light chain can then be tested on the two different heavy chains of interest.
In another approach, a light chain might be selected by assaying a phage display library of light chain variable sequences using the heavy chain variable sequences of both heavy chains of interest as probes. A light chain that associates with both heavy chain variable sequences might be selected as a light chain for the heavy chains.
In another approach, a candidate light chain might be aligned with the heavy chains' cognate light chains, and modifications are made in the light chain to more closely match sequence characteristics common to the cognate light chains of both heavy chains. If the chances of immunogenicity need to be minimized, the modifications preferably result in sequences that are present in known human light chain sequences, such that proteolytic processing is unlikely to generate a T cell epitope based on parameters and methods known in the art for assessing the likelihood of immunogenicity (i.e., in silico as well as wet assays).
All of such approaches rely on in vitro methods that subsume a number of a priori restraints, e.g., sequence identity, ability to associate with specific pre-selected heavy chains, etc. There is a need in the art for compositions and methods that do not rely on manipulating in vitro conditions, but that instead employ more biologically sensible approaches to making human epitope-binding proteins that include a common light chain.
In addition, therapeutic antibodies, e.g., bispecific therapeutic antibodies, have some limitations in that they often require high doses to achieve desired efficacy. This is partly due to the fact that antibody-antigen complexes are internalized into the endosome, and are targeted for lysosomal degradation in a process called target-mediated clearance. Thus, there is a need in the art for methods and compositions that lead to more efficient antibody recycling, e.g., bispecific antibody recycling, and prevent degradation of the antibody by promoting dissociation of antibody-antigen complexes in the endosomal compartment without compromising the specificity and affinity of the antibody toward the antigen.
Drugs administered into the body, including therapeutic monoclonal antibodies, can be affected via various elimination mechanisms, including glomerular filtration (e.g., into urine), secretion (e.g., into the bile), and catabolism by cells. While small molecules are cleared from the body via renal filtration, the majority of secreted antibodies (e.g., IgG, which are too big to be filtered through glomeruli) are primarily removed from the body via cell-mediated catabolism, e.g., fluid-phase endocytosis (phagocytosis) or receptor-mediated endocytosis. For example, soluble molecules with several repeated epitopes are bound by a plurality of circulating antibodies, and the resulting large antigen-antibody complexes are phagocytosed rapidly into cells for degradation. On the other hand, cell surface target receptors, which are bound by antibodies (i.e., receptor-antibody complexes), undergo target-mediated endocytosis in a dose-dependent manner, which leads to formation of endosomes destined for lysosomal degradation inside cells. In some cases, the endocytosed receptor-antibody complexes bind neonatal Fc receptors (FcRn) inside the endosomes in a pH-dependent manner and are routed back to the cell surface for release into plasma or interstitial fluids upon exposure to a neutral extracellular pH (e.g., pH 7.0-7.4).
There is a need in the art for systems, e.g., non-human animals, cells, and genomic loci that generate antigen-binding proteins with titratable residues, e.g., genetically modified loci that rearrange immunoglobulin gene segments to generate heavy chain variable domains that respond to changes in pH, e.g., that donate or accept protons and, e.g., whose binding characteristics differ according to protonation state.
There is also a need in the art for methods and compositions that can further increase recycling efficiency of endocytosed antigen-binding proteins by promoting dissociation of antigen-binding proteins from receptor-antigen-binding protein complexes or by increasing the affinity of antigen-binding proteins toward FcRn in an acidic endosomal compartment without compromising the specificity and affinity of the antigen-binding protein toward an antigen of interest.